The subject matter disclosed herein relates to an all-solid-state reflection-controllable electrochromic device. More particularly, the subject matter disclosed herein relates to an all-solid-state reflection-controllable electrochromic device that has high transmittance in the bleached state and is capable of switching in a short period of time over a large surface area between a transparent state and a reflective state.
Windows and other openings are generally the place where the most heat enters and escapes buildings. For example, during the winter about 48% of the heat produced by a heating system of a building escapes through windows of the building. During the summer, the proportion of heat that enters an air-conditioned room through the windows can reach about 71%. A tremendous energy savings can, therefore, be realized by effectively controlling light and heat entering and escaping through windows. Light-control glass has been developed to control the bi-directional flow of light and heat through a window.
There are several ways that light is controlled by light-control glass. One way is to form an electrochromic material on the glass in which the transmissivity of the electrochromic material reversibly changes under application of a current or a voltage. Another way is to form a thermochromic material on the glass in which the transmissivity of the thermochromic material changes with temperature. Yet another way is to use a gastrochromic material that changes its transmissivity by controlling the atmosphere gas. Of these, electrochromic-based light-control glass has been researched in which a tungsten-oxide thin film is used for the light-control layer. Some commercial products based on this type of electrochromic light-control glass have already appeared.
Conventional electrochromic-based light-control glass, including tungsten-oxide-based versions, all control light by absorbing the light using a light-control layer. A significant drawback with absorbing the light is that heat is produced and radiated into a room when the light-control layer absorbs light, thereby diminishing the energy-saving effect of the conventional electrochromic light-control glass. To eliminate this drawback, another approach of reflecting light rather than absorbing light has been considered. Accordingly, a material capable of reversibly switching between a mirror state and a transparent state would be useful.
For a long time, such a material capable of switching between a mirror state and a transparent state was not found, but in 1996 a group in the Netherlands discovered a hydride of a rare earth, such as yttrium or lanthanum, switches between a mirror state and a transparent state under the influence of hydrogen. Such a material is conventionally referred to as a “switchable mirror”. See, for example, J. N. Huiberts et al., Nature, 380, 1996, 231. The rare-earth hydrides undergo a large change in transmissivity, and have excellent light-control mirror characteristics. Nevertheless, because a rare-earth element is used in the material, there are problems in terms of resources and cost when rare-earth-hydride-based switchable mirrors are used for window coatings and other applications.
Additionally, conventional metal-hydride-based mirrors suffer from poor cycle life due to the reactive nature of the metal film, which is readily attacked by air or water. Notably, water is one component of the electrolyte in electrochromic hydride mirrors, and may be produced during removal of hydrogen from the mirror film in both electrochromic and gasochromic devices. The life-cycle degradation is conventionally inhibited by using additional barrier layers for protecting the active materials and by sealing devices for preventing access of environmental air and water. The former approach of adding barrier layers is difficult to achieve and may not be effective after long periods of use. The latter approach of sealing does not address the problem of internal sources of water or oxygen.
More recently, U.S. Pat. No. 6,647,166 B2 to T. J. Richardson discloses alloys of magnesium and transitional-metals that can be used as switchable-mirror materials, thereby significantly reducing the cost of materials for electrochromic-based light-control glass.
Additionally, the commonly used transparent conducting oxide (TCO) coatings, such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and aluminum-doped zinc oxide, are unstable at the switching voltages used for the reflection-controllable electrochromic devices. Side reactions, such as irreversible lithium intercalation, occur at the extremely low voltages, resulting in fast degradations of the TCO layers. Therefore, mirror electrodes for reflective-type electrochromic device prepared to date have relied upon the intrinsic conductivity of the active material. Volume expansion and contraction during cycling contribute to loss of connectivity within the electrode and eventually produce isolated regions that switch slowly or not at all.
U.S. Pat. No. 7,042,615 B2 to T. J. Richardson discloses use of a semi-metal, antimony, as a switchable-mirror thin-film material that is based on lithium insertion and extraction. Addition of low-resistivity metals, such as Ag and Cu, to the thin-film material is also disclosed by Richardson for improving the cycling stability of the mirrors. The added low-resistivity metals reduce the volume change and increase the conductivity of the switchable-mirror thin-film materials in all states of charge, thereby improving the uniformity of the current density across the thin film and reducing the stress between regions of different composition. For an antimony film containing 33% copper, the volume change on full lithiation is about 70% as opposed to 136% for pure Sb. The resulting semiconducting phase is diluted by copper and, therefore, less prone to coarsening on delithiation. Nevertheless there exists a serious problem with noble metals, such as Ag and Cu, if used as an additive. For example, when noble metals are added in the active layer of an all-solid-state electrochromic device, the noble metals can easily migrate (referred to herein as “electromigration”) into the solid electrolyte layer and create shorts for the device (and, thus, a failed device), particularly when an electric field is applied to the device to switch the state of the device, and the solid electrolyte layer of the device is relatively thin (for example, less than about 100 nm in thickness).